602 
DR. J. T. ROTTOMLEY OX THERMAL 
where v is the temperature of the cooling globe, i^Qthat of the enclosure, c the capacity 
for heat of the globe, and S its surface, while e is a co-efl&cient wdiich has been called 
the emissivity. It is easy to show by an application of Fourier’s equations that, to 
a first approximation, sufficient for my present purpose, the loss by conduction by the 
wdres may be allowed for by adding to S, or I should rather say to eS, a small 
quantity depending on the diameter, conductivity, and emissivity of the wure, the 
length of the wires being so great that no heat passes away by conduction at the 
ends. The numerical value of the quantity to be added to cS I have not yet 
determined exactly ; but I have made an estimate of it and applied it as a correction 
in obtaininof the results given below. 
Another interesting point requiring consideration is the following :—The arrange¬ 
ments explained above give approximately the temperature at or near the centre of 
the copper globes. Those of Mr. D. Macfarlane did the same in the case of his 
experiments. In a criticism of Macfarlaxe’s results, by M. CoRXU, ‘Journal de 
Physique,’ December, 1873, an objection was taken to the thermo-electric method as 
compared w'itli the method used by Duloxg and Petit, w’here the bulb of a thermo¬ 
meter wms the cooling body, on the ground that the temperature of the centre may 
be different from that of the surface. This objection was completely answered by 
numerical calculations founded on the dimensions and conductivities of the globes in 
question ; and, in an interesting appendix the whole Cjuestion is discussed in the 
‘ Poyal Society Proceedings,’ June 10, 1875.^ 
With these explanatory remarks I proceed to a brief statement of the results 
obtained, but I wash it to be clearly understood that I do not yet consider myself 
entitled to draw general conclusions as to laws of radiation from the observations 
completed and calculated down to tlie present date.* * * § ". Theories of radiation have, m 
recent years, been discussed by Stef ax, t PI. F. Weber,! Ferrel,§ and others; but 
it apjiears to me that the main difficulty of making any satisfactory theory is the 
want of experimental data on which to found a theory, and by wdiich to test it. I 
venture to think that the data required are still far from sufficient; and, indeed, 
this is admitted and pointed out as a difficulty by the most recent of these w'riters, 
the late Mr. Ferrel. 
I have experimented solely on the copper globes wdth their surfaces prepared by 
sooting and silvering. The sooting was done very carefully with a gas flame or 
wdth paper or cotton waste impregnated with powxlered resin, or powxlered camphor, 
or moistened wdth turpentine. The endeavour was to put on a thoroughly con¬ 
tinuous but thin coating of lamp-black, and thus to obtain wdiat I presume w'e may 
take to be a surface giving maximum of radiation. On the other hand the silvered 
* Republished iu ‘ Lord Kelyix’s Collected Papers,’ vol. 3, p. 245. 
t Stefan, ‘ Bericbte der Wiener Akademie,’ vol. 79, 1879. 
t Professor H. F. Weber, ‘Akademie der Wissenscliaften zu Berlin.’ Berlin, 1888. 
§ W. Ferrel, ‘ American Journal of Science,’ July, 1889, and Feln-uarv, 1890. 
